1. Field of the Invention
This invention relates to an active sound reduction apparatus, and an active noise insulation wall having it. More specifically, the invention relates to an active sound reduction apparatus which is laid along highways, ordinary roads, and railways, and which is useful in insulating noises caused by traveling vehicles, trains, etc. as sound sources.
2. Description of the Related Art
To insulate noise from a sound source, such as a vehicle or a train traveling on a highway, an ordinary road, or a railway, a noise insulation wall is erected along such a highway or the like. In recent years, an active acoustic control cell has been developed as an effective insulator of noise produced in such a case. The active acoustic control cell senses a sound from a sound source by a microphone, and processes an electric signal based thereon to generate a sound from a speaker so that a sound pressure at a predetermined position is reduced to zero, thereby reducing noise which is propagated after diffraction from the sound source to the outside of a noise insulation wall. That is, this type of active acoustic control cell is disposed on an upper end surface of the noise insulation wall, a vertical wall provided along a road or the like. This active acoustic control cell performs control in such a manner as to decrease a diffracted sound pressure component (at the upper end surface) of coming noise by active means (see, for example, Japanese Unexamined Patent Publication No. 1997-119114).
FIG. 27 is an explanation drawing conceptually showing an example of an active noise insulation wall having such an active acoustic control cell. As shown in the drawing, a plurality of the active acoustic control cells A are disposed on an upper end surface of a noise insulation wall B, a vertical wall, along a longitudinal direction of the noise insulation wall B. The active acoustic control cell A has a structure in which a speaker 2 being a sound wave generator, an amplifier 3, a skin material 4, a microphone 5 being a sound detector, and a control circuit 6 are integrated into a casing 1. The speaker 2 is opposed to the skin material 4 so that a sound wave generated by the speaker 2 is incident on the skin material 4. The microphone 5 is installed at a position between the skin material 4 and the speaker 2. Thus, the speaker 2 outputs an electric signal corresponding to a sound wave detected by the microphone 5. Based on the electric signal, the control circuit 6 performs predetermined computation, and issues a control signal obtained thereby to the amplifier 3. The amplifier 3 sends a drive signal corresponding to the control signal to the speaker 2. The speaker 2 generates a sound wave corresponding to the drive signal. Transfer characteristics G based on the characteristics of the speaker 2, amplifier 3, microphone 5 and control circuit 6 is adjusted to negative infinity or a value close to negative infinity, or −1, or a value close to −1, so that control is performed over a broad range of frequencies. That is, the control circuit 6 stores a pattern of the transfer characteristics G at each frequency, performs required computations in response to electric signals sent from the microphone 5, and feeds predetermined control signals to the amplifier 3. The transfer characteristics G is controlled in this manner. Thus, if the sound pressure acting on the microphone 5 is designated as P, and a control sound pressure produced by the speaker 2, as Pc, then Pc=G·P holds. As a result, the sound pressure of a diffracted sound originating from a noise source (e.g., a driveway side), changing in direction at the upper end surface of the noise insulation wall B upon diffraction, and leaking to an opposite side of the noise insulation wall B (e.g., a private house side) can be decreased.
FIG. 27 shows an example of only one row of the active acoustic control cells A disposed along the noise insulation wall B. There is no restriction on the number of rows of the active acoustic control cells A. The number of rows of the active acoustic control cells A can be determined, as desired, according to the level of the noise to be decreased. An active noise insulation wall according to an earlier technology, having three rows of the active acoustic control cells A disposed thereon, is shown in FIG. 28. As the drawing shows, in this type of active noise insulation wall, three of the active acoustic control cells A are arranged in a horizontal direction perpendicular to a longitudinal direction of the noise insulation wall B, without spacing between the adjacent active acoustic control cells.
In the active noise insulation wall according to the earlier technologies, as described above, it induces a cost increase to broaden the frequency band targeted by the active acoustic control cell, or to provide a plurality of the active acoustic control cells. That is, the conventional active noise insulation wall is not sufficient for reducing nose effectively at a low cost.